This project involves 1) ongoing studies assessing the role of CD4+ T cells in rejection of MHC class I disparate skin grafts 2) new studies that assess the role of the macrophage in skin allograft rejection, and 3) new studies assessing the role of primed lymphocyte populations in graft rejection. 1) To assess the role of CD4+ T cells in rejection of MHC class I disparate Kb mutant skin grafts, two different types of experiments have been performed: antibody depletion studies in which thymectomized or non- thymectomized mice were depleted of CD4+ T cells in vivo by injection of antibody and engrafted with MHC class I disparate skin and adoptive transfer studies in which graded numbers of CD4+ T cells are added to athymic nude mice reconstituted with CD8+ T cells. Both of these approaches have yielded compatible information pointing to inhibition of MHC class I disparate graft rejection by CD4+ T cells. Further studies assessing the specificity of the effect are planned as well as extension of these studies to non-Kb mutant MHC class I disparate skin grafts. 2) Macrophages are present in abundance in skin grafts rejected across MHC class II and minor-histocompatibility barriers but not in those rejected across MHC class I barriers. Although the activity of macrophages in the rejection process has not been defined, it is possible that they contribute in a multiplicity of ways, including enhanced antigen presentation, elaboration of cytokines and other factors, and direct induction of tissue damage. Studies to date indicate that macrophages present in rejecting MHC class II disparate grafts are activated and that cytokines which may be of macrophage origin (IP-10 and IL-1) are elaborated. Further studies are planned to establish the source of the cytokines. Additional studies in which macrophages are depleted by toxic liposomes will most directly assess their role in rejection across differing histocompatibility disparities. 3) Although the cellular populations mediating allograft rejection have begun to be understood for primary rejection responses, almost nothing is known about the cellular mechanisms and interactions involved in secondary (primed) responses. Studies in which the specificity of the effector mechanisms, the phenotype of the cellular populations infiltrating rejecting grafts, and the cytokine activity present in rejecting grafts have commenced.